This invention relates to apparatus for measurement of internal and external dimensions of a non-opaque hollow fiber. More particularly, the apparatus determines hollow fiber dimensions from the electromagnetic radiation absorption of the fiber.
Hollow fibers have been used as membranes in a variety of separation processes. U.S. Pat. Nos. 3,228,876, 3,339,341, 3,422,008, 3,423,491, 3,703,962, 3,755,034, 4,364,759 and 4,421,529 represents a very small sample of the prior art pertaining to hollow fiber membranes. The external dimensions, wall thickness and bore size of the hollow fiber have a direct impact on its performance as a membrane. Consequently, accurately and easily to ensure uniform membrane characteristics.
The external dimensions of a solid fiber are conveniently measured optically. J. R. Dees et al, J. Appl. Poly. Sci., 18, pp. 1053, 1055 (1974), report that the diameter of solid fibers can be determined from photographs of a projected image. Lasers have also been used in devices to measure external dimensions of fibers as well as other shapes. However, the bore diameter and wall thickness of hollow fibers have most commonly been determined by examination of magnified fiber sections. This procedure is not only tedious, but prone to inaccuracies due in part to the fiber handling required. Further, no on-line procedure for measurement or continuous monitoring of hollow fiber dimensions during fiber spinning has been available.
It has now been found that hollow fiber dimensions can be determined from the differential light absorption of the fiber. This fact is surprising since the effects of reflection, refraction and diffraction upon light incident on the hollow fiber would be expected to result in a very complex absorption pattern due to scattering and other effects. In this regard, it is noted that initial efforts to determine hollow fiber dimensions using a laser as a light source resulted in a complex absorption pattern.